Gamma compensating circuit

ABSTRACT

A circuit having a nonlinear response characteristic provides gamma compensation to a video luminance signal and matches any desired output response curve by selecting the number of diodes connected as series-connected circuits in the compensation circuit. The relationship among the various numbers of series-connected diodes is dependent upon having a current flowing into an output circuit which is the same as a current flowing out of a collector circuit of a differential amplifier, to which the input video luminance signal is applied, and through series-connected diode circuit. These currents are maintained equal by use of a current-mirror circuit, which provides an input current to an output transistor that has in its emitter circuit one of the series-connected diode circuits, and the compensated luminance signal flows in the collector circuit thereof. The differential amplifier is stabilized and balanced by a difference amplifier connected in feedback across the collector leads of the differential amplifier in conjunction with a pedestal clamping synchronization signal and a capacitor to provide a balancing signal at the base lead of the second transistor of the differential transistor amplifier.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to circuits having a nonlinear responsecharacteristic and, more particularly, to a circuit having a nonlinearresponse characteristic for use in providing gamma compensation in avideo camera.

2. Description of the Prior Art

It is well known that a color picture tube has a nonlinear transfercharacteristic, however, the pick-up tube in a video camera hasessentially a linear response characteristic. Therefore, in order toproduce a pleasing color picture, the transfer characteristic of thepick-up camera must be compensated in the reciprocal manner relative tothe picture tube characteristic. Such compensation is typically referredto as gamma compensation, in which gamma is understood to be the slopeof a log-log plot of the light transfer characteristic.

Gamma-compensation circuits are designed using a number ofseries-connected diodes to provide the required nonlinear responsecharacteristics, in which the principal that the voltage across a diodeis approximately proportional to the square root of the current throughsuch diode is advantageously employed. Some known gamma-compensationcircuits are temperature dependent and, thus, this characteristicpresents a drawback to the use of such circuits. Other known gammacompensation circuits have a direct current component in the nonlineartransfer function, which adversely affects the stability of the gammacompensation circuit.

Other known techniques for constructing gamma-compensation circuitsemploy using several gamma-compensating circuits in series, so thatdifferent portions of the nonlinear curve can be provided by different,individual, compensation circuits. Nevertheless, matching of thesecircuits is quite difficult and this technique has generally been noteffective.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide acircuit for providing gamma compensation which can eliminate theabove-noted defects inherent in the prior art.

Another object of this invention is to provide a gamma-compensationcircuit that is essentially temperature independent and is stable forall practical temperatures.

A further object of this invention is to provide a gamma compensationcircuit in which no DC is present in the non-linear portion of thecircuit so that the gamma compensation circuit is stable regardless ofthe input waveform.

In accordance with one aspect of the present invention, gammacompensation is provided using a current source and plurality of diodesto provide the basic gamma compensation curve, however, the luminancesignal that comprises the signal to be gamma compensated is fed to adifferential amplifier formed of a transistor pair and having one ormore diodes connected in series between the collectors, with onecollector being connected to ground through another series-connecteddiode circuit. The luminance signal is fed to one base lead of thisdifferential transistor amplifier and the other base lead is clamped atthe pedestal level of the luminance signal by a capacitor and feedbackcircuit that sets both collector voltages at the same level. This systemeliminates any DC voltage levels present in the actual luminance signalcurrent and by using a current mirror circuit a gamma-compensationcharacteristic represented by one of several different exponentialpowers is available.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following detailed descriptionof illustrative embodiments thereof to be read in conjunction with theaccompanying drawings, in which the same reference numerals identify thecorresponding elements and parts in the several views.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a gamma-compensation circuit known inthe prior art;

FIG. 2 is a circuit diagram of a gamma-compensation circuit according toan embodiment of the present invention;

FIG. 3 is a graph of a gamma-compensated output signal produced by theinventive circuit of FIG. 2;

FIG. 4 is a circuit diagram of a gamma compensation circuit according toanother embodiment of the present invention; and

FIG. 5 is a circuit diagram of a further embodiment of agamma-compensation circuit according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A known gamma-compensation circuit that provides the required non-lineartransfer characteristic is shown in FIG. 1, in which a constant currentsource Q₂, series-connected diodes D₁, and series-connected diodes D₂are all arranged in series between a voltage supply terminal T₁ andground or reference potential. The numbers of diodes D₁ and D₂ in eachseries-connected circuit are represented by N-K and K, respectively. Atthe junction between the first circuit of series-connected diodes D₁ andthe second circuit of series-connected diodes D₂ the output of aconstant-current source Q₁ is connected. The input of theconstant-current source Q₁ corresponds to the video luminance signal,and another constant-current source Q₃ is connected to ground potentialin parallel with the series-connected diode circuits D₂. Also connectedbetween the supply voltage terminal T₁ and ground potential is thecollector-emitter junction of transistor Q₄, a third series-connecteddiode circuit D₃, and another constant-current source Q₅. The base drivefor transistor Q₄ is provided at the junction of the output ofconstant-current source Q₂ and the input of the first diode circuits D₁.The output of the gamma-compensation circuit of FIG. 1 is taken fromtransistor Q₆ that has a base lead connected at the junction formed atthe end of the third diode circuit D₃, and the input to constant-currentsource Q₅. Transistor Q₆ is further connected with its collector leadconnected to ground through a fourth circuit of series-connected diodesD₄. The number of diodes D₄ in this fourth circuit is represented asJ-1.

Analyzing the operation of the gamma-compensation circuit of FIG. 1, itis seen that current i₁ represents the input luminance signal currentthrough constant-current source Q₁ and i₆ is the output signal flowingin the collector circuit of transistor Q₆. I₂ is the current flowing dueto constant-current source Q₂, current I₃ is the current flowing due toconstant-current source Q₃, and current I₅ is the current flowing due toconstant-current source Q₅. The saturation current of diodes D₁ -D₄,inclusively, is represented by I₅, as is the base-emitter path currentof transistors Q₄ and Q₆. The variables used in defining the number ofdiodes in each of the various series-connected diode circuits, that is,N, J, and K are all positive integers equal to or larger than 1. It isthe numbers of these diodes that determine the shape of the nonlinearcompensation curve.

Therefore, assuming that:

    I.sub.2 =I.sub.3                                           (1)

only the input signal current i₁ flows through the diode circuit D₂ and,because the terminal voltage of the series-circuit made up of diodes D₁and D₂ must be the same as the terminal voltage of the series circuitsof the base-emitter paths of transistors Q₄ and Q₆ and diode circuits D₃and D₄, respectively, then the following equations can be obtained.

    (N-K)(kT/q)ln(I.sub.2 /I.sub.s)+K(kT/q)ln(i.sub.1 /I.sub.s)=(N-J)(kT/q)ln(I.sub.5 /I.sub.s)+J(kT/q)ln(i.sub.6 /I.sub.s) (2)

and, thus:

    (I.sub.2 /I.sub.s).sup.N-K ·(i.sub.1 /I.sub.s).sup.K =(I.sub.5 /I.sub.s).sup.N-J ·(i.sub.6 /I.sub.s).sup.J      (3)

    i.sub.6 =αi.sub.1.sup.K/J                            (4)

where:

    α=[I.sub.2.sup.(n-k)/j /I.sub.5.sup.(N-J/J ]         (5)

and

k=Boltzman's constant;

T=absolute temperature in K°; and

q=charge of an electron in electron volts

Therefore, as evidenced by equation (4), the various different gammacharacteristics can be obtained by selecting different values of J and Kwhich correspond to the number of diodes D₁, D₂, D₃, and D₄, utilized inthe circuit. Note particularly that equation (4) does not contain anyterm that is temperature dependent and, accordingly, the temperaturestability characteristics of the gamma compensation circuit shown inFIG. 1 are relatively good. Nevertheless, in the circuit of FIG. 1equation (1) was assumed in order to derive equation (4) and if it isnot possible to obtain the situation of equation (1), then equation (4)becomes

    i.sub.6 =α(I.sub.2 -I.sub.3 +i.sub.1).sup.K/J        (6)

Thus, the desired gamma-compensation characteristic can not be obtainedbecause there is a DC component (I₂ -I₃) contained in equation (6). ThisDC component (I₂ -I₃) is the difference between the direct currents I₂and I₃ and affects the initial rise-time portion of the low-level signalof the gamma-compensation characteristic, and to eliminate this DCcomponent currents I₂ and I₃ are required to equal each other at the lowcurrent level, that is, at the commencement of the current wavecorresponding to the pico-ampere region of the current response curve.As might be expected, it is very difficult to have two current waveformscoincide exactly within such close tolerances as the pico-ampere rangeand, thus, considering the temperature characteristic, stable operationof the gamma-compensation circuit of FIG. 1 can not realistically beexpected. Furthermore, since the voltage at the junction of diodecircuits D₁ and D₂ decreases as the gamma-compensation curve rises, itis difficult to monitor or detect the current I₃ in a stable fashion.

One approach which may appear possible is to create the desiredgamma-compensation characteristic curve by combining a first gammacharacteristic and a second gamma characteristic, that is, by adding twodifferent output signals from two gamma-compensation circuits. However,this approach is not particularly feasible, since if the currents I₂ andI₅ of the circut that produces the first desired gamma characteristicand the currents I₂ and I₅ of the second gamma characteristic circuitare not equal, respectively, the co-efficient α, as represented inequations (4) and (5) will be different and, therefore, it is notpossible to combine the two circuits to produce the desiredgamma-compensation characteristic.

An embodiment of the present invention shown in FIG. 2 overcomes theabove problems and eliminates DC currents that might be present in thegamma-compensation circuit. Specifically, a differential amplifier 11 isprovided at the input of the gamma-compensation circuit and resistorsR₁₁ and R₁₂ are connected in series between the emitter leads ofrespective transistors Q₁₁ and Q₁₂. The junction of resistors R₁₁ andR₁₂ is connected to ground potential through constant-current sourceQ₁₃, and these circuit elements combine to form the differentialamplifier 11. The input signal, which may be a video luminance signal,is applied at input terminal T₁₁ and fed to the base lead of transistorQ₁₁. The collector leads of transistors Q₁₁ and Q₁₂, respectively, areconnected to a bias supply voltage V_(cc) at terminal T₁ throughconstant-current sources Q₁₄ and Q₁₅, respectively. Additionally, thecollector circuits of transistors Q₁₁ and Q₁₂ are connected together byseries connected diodes D₂, of a number represented by the variable K.The collector circuit of transistor Q₁₁ is connected to ground potentialthrough a series connected diode circuit D₁, the number of diodes inwhich is represented by N-K.

The base circuit of transistor Q₁₂ is driven by the output of adifference amplifier 12 that is connected at its plus and minus inputsto the collector circuits of transistors Q₁₂ and Q₁₁, respectively. Theinput lead of transistor Q₁₂ is voltage clamped by capacitor C₁₁ that isconnected between the base lead of transistor Q₁₂ and ground potential.The voltage difference amplifier 12 driving the base of transistor Q₁₂is supplied with clamping sync pulses at terminal T₁₂ that are in timewith the DC pedestal level of the video luminance signal fed in atterminal T₁₁.

The constant-current source Q₅ of FIG. 1 is embodied in part in theinventive circuit of FIG. 2 by transistor Q₅ and output transistors Q₄and Q₆ and series-connected diode circuits D₃ and D₄ are connected inthe circuit of FIG. 2 as discussed hereinabove in regard to FIG. 1. Thebase circuit of transistor Q₄ is driven by the collector circuit oftransistor Q₁₂, and the base lead of transistor Q₅ is connected to ajunction between diodes in the series-connected diode circuit D₁.Specifically, base lead of transistor Q₅ is connected to the junction ofthe first diode D_(1a) and the second diode D_(1b) of diode circuit D₁,and the other end of diode D_(1a) is connected to ground potential.Diode D_(1a) and transistor Q₅ comprise current mirror circuit 13, theimportance of which to the present invention will be seen hereinbelow.Transistors Q₄₁, Q₅₁, Q₆₁ and diode circuits D₃₁ and D₄₁ correspond totransistors Q₄, Q₅, and Q₆ and diode circuits D₃ and D₄, respectively,and are connected in the same fashion. Note that diode D_(1a) andtransistor Q₅₁ comprise current-mirror circuit 131.

In the inventive circuit of FIG. 2, currents I₁₃, I₁₄, and I₁₅ producedby constant current sources Q₁₃, Q₁₄, and Q₁₅, respectively, may havetypical values of: I₁₃ =400 micro amps; I₁₄ =200 micro amps; and I₁₅=300 micro amps. The positive luminance input signals applied atterminal T₁₁ results in the signal current component flowing in a loopformed by the differential amplifier as indicated by arrow i₁ and inwhich the loop is comprised of transistor Q₁₁, resistors R₁₁, R₁₂,transistor Q₁₂, and diode circuit D₂. When the level of the input signalgoes to zero, that is, during the time when the input signal is on itspedestal level, as represented by the dashed p line relative to thewaveform applied at input terminal T₁₁, the voltage comparator ordifference amplifier 12 works to equalize the collector voltages oftransistors Q₁₁ and Q₁₂, so that no direct current component can flowthrough diode circuit D₂. That is, when the input luminance signal is atthe pedestal level, the comparator 12 is operative by way of theclamping sync pulse fed in at terminal T₁₂. Clamping is well known andthe sync pulses are generally available, since clamping is necessary toprovide a reference for reinsertion of DC voltage levels which are lostwhen the luminance signal is passed through RC-coupled stages. Thedifference amplifier 12 compares the collector voltages of transistorsQ₁₁ and Q₁₂ and produces a comparison output voltage signal that isclamped by clamping capacitor C₁₁ and fed to the base circuit oftransistor Q₁₂. Thus, the collector voltages of transistors Q₁₁ and Q₁₂are forced to be equal, no direct current can flow through diode stringD₂ and the only current signal present is the input signal component i₁.

Therefore, assuming that the only current flowing throughseries-connected diode circuit D₁ is current I₂ then the followingcurrent equation can be made:

    I.sub.2 =I.sub.14 +I.sub.15 -I.sub.13                      (6)

Thus, the constant element of direct current I₂ flows through diodecircuit D₁, and no current component of the input signal I₁ flows inthis diode string. Furthermore, the series-connected diode circuits D₁and D₂ are connected in the base circuit of transistor Q₄. Based uponthe above it is seen that the gamma compensation circuit of FIG. 2 issubstantially the equivalent functionally to the circuit of FIG. 1,except that the DC component has been removed. Thus, equation (4)hereinabove applies equally to the circuit of FIG. 2 and moreover thesame equation relative to current i₆ applies as follows:

    i.sub.6 =αi.sub.1.sup.K/J 1                          (8)

Where the number of diodes in diode circuit D₄₁ equals J₁ minus one (J₁-1). Thus, both current i₆ and i₆₁ provide gamma-compensationcharacteristics relative to the input signal current i₁.

Current-mirror circuits 13 and 131 are formed of transistors Q₅ anddiode D_(1a) and transistor Q₅₁ and diode D_(1a), respectively, and thecollector currents I₅ and I₅₁ of transistors Q₅ and Q₅₁, respectively,can be represented by the following:

    I.sub.5 =I.sub.2 =I.sub.51                                 (9)

Therefore, the equations (4) and (8) hereinabove can be rewritten asnormalized by currents I₂ as follows:

    (i.sub.6 /I.sub.2)=(i.sub.1 /I.sub.2).sup.K/J              (10)

    (i.sub.61 /I.sub.2)=(i.sub.1 /I.sub.2).sup.K/J 1           (11)

Thus, equations (10) and (11) represents the output currents I₆ and I₆₁that have been standardized or equalized by the current I₂ and whichproduce exponential functions of the input signal i₁ to the power K/Jand K/J₁, respectively.

FIG. 3 represents the gamma-compensation characteristics of the circuitof FIG. 2 and typically shown are the curves representing thecompensated currents I₆ and I₆₁. Note that if the two currents are addedthen a different compensation characteristic would be provided. Equation(4) indicated that a desired gamma-compensation was possible and thecurves of FIG. 3 bear this out. Additionally, note that while thecircuit of FIG. 1 could provide the desired gamma-compensationcharacteristic curve it was not always possible, whereas the circuit ofFIG. 2 does not depart from the desired gamma-compensation curve butprovides it in stable fashion.

Another embodiment of the present invention is shown in FIG. 4 in whichthe compensated output signals are exponentially compensated todifferent powers of the input signal, specifically, the exponentialpower of one, one-half, and one-fourth. In this embodiment the numbersof diodes are selected based upon the following parameters: N=4, K=1,J=1, and J₁ =2. Transistors Q₄₂, Q₅₂, and Q₆₂ and diode D₃₂ areconnected in the exact same fashion as were transistors Q₄, Q₅, and Q₆and diode D₃, respectively, in FIG. 2. Diode circuit D₄₂ is providedwith a number of diodes given by J₂, which in this example equals 1.Resistors R₁₃ and R₁₄ and diode D₁₁ provide a voltage offset to theinput signal i₁ and also decrease the input impedance as seen across theinputs of integrated-circuit difference amplifier 12, which is connectedacross the collector circuits of the differential-amplifier transistorpair Q₁₁ and Q₁₂, and thereby improves the frequency responsecharacteristics. Otherwise, the gamma-compensation circuit of FIG. 4operates in the same fashion as described in relation to FIG. 2, withthe exception that the additional exponential powers are made possibleby the added output stages.

In the gamma compensation circuits of FIGS. 2 and 4 since suchcompensation is provided by the series-connected diode circuits, whenthe number of such diodes changes, the temperature characteristics ofthe output current I₆ or I₆₁ will also change. In the embodiment of thepresent invention shown in FIG. 5, this changing of the temperaturecharacteristic of the output current I₆ is eliminated.

Equation (10) above that described the embodiment of FIG. 2 can berewritten as:

    i.sub.6 =I.sub.2.sup.1-(K/J) i.sub.1.sup.K/J               (12)

provided that the temperature characteristics of currents I₁ and I₂ aredifferent such that current I₁ will change to (1+a)I₁ and current I₂will change to (1+b)I₂. Thus, when the temperature changes by anincrement ΔT, equation (12) will become: ##EQU1## Equation 13 thenrepresents that the temperature characteristic of the compensated outputcurrent I₆ will change according to the number of diodes K and J and,moreover, this exact same temperature compensation characteristic willbe present for current I₆₁, as well. Furthermore, because thetemperature characteristics of the compensated output currents I₆ andI₆₁ depend upon the number of diodes K, J and K, J₁, respectively, thetemperature characteristics of current I₆ will differ from thecharacteristic of current I₆₁ when J does not equal J₁. As a result ofthis, the sum of the output currents of the circuit shown in FIG. 2 orin FIG. 4 can not be standardized because of the difference oftemperature characteristics among and between the plurality of outputsignals. Nevertheless, this is standardized in the circuit of FIG. 5, inwhich current I₂ that flows through the diode circuit D₁ is detected andthe detected signal is fedback to the current source Q₁₄ to maintaincurrent I₂ at a reference value I_(r).

In the embodiment of FIG. 5 there are three diodes D₁ provided,specifically, diodes D_(1a), D_(1b), and D_(1c) and a single diode D₂interconnecting the collector leads of the differential amplifier inputcircuit and two diodes D₃ connected in the output circuit formed bytransistor Q₄. There is no diode D₄ in this embodiment in the emittercircuit of the output transistor Q₆, and single diodes are eachconnected in the collector circuits of transistors Q₄₁ and Q₆₁. FromFIG. 5 and the above it is seen that the parameters utilized in thecircuit of FIG. 5 are: N=4, K=1, J=1, and J₁ =2. As in the embodiment ofFIG. 4, resistors R₁₀₁ and R₁₀₂ and diode D₁₀₁ are connected in thecollector circuits of differential amplifier transistors Q₁₁ and Q₁₂,respectively, and an additional resistor R₁₀₃ is also provided in thebase input circuit of transistor Q₄. These additional circuit elementsare provided for the same reason as in the embodiment of FIG. 4, thatis, to provide a voltage offset to input luminance signal current I₁ andalso to decrease the input impedance seen by the inputs of integratedcircuit comparator 12, as across the collectors of the differentialtransistor pair Q₁₁ and Q₁₂, thereby improving the frequencycharacteristics of the entire circuit.

A current-mirror circuit is connected in the collector circuit oftransistor Q₁₂ and includes transistors Q₁₅₁, Q₁₅₂, and Q₁₅₃ that areconnected to be driven by constant-current source Q₁₅₄. TransistorsQ₁₅₁, Q₁₅₂, and Q₁₅₃ that make up current-mirror circuit Q₁₅ areconnected in the conventional fashion and the appropriate bias voltageis obtained by connections in the emitter circuits to the bias voltageV_(cc). Note that this is a specific embodiment of constant-currentsource Q₁₅ employed in the embodiments of the invention shown in FIGS. 2and 4. Similarly, the current-mirror circuit Q₁₄ connected to thecollector circuit of transistor Q₁₁ comprises transistors Q₁₄₁, Q₁₄₂,and Q₁₄₃ connected in the conventional fashion and are also connected tothe bias voltage V_(cc) through respective emitter resistors.Current-mirror circuit Q₁₄ is then operably connected to differentialamplifier 16 formed of transistors Q₁₆₁ and Q₁₆₂, the collectors ofwhich are both connected to ground potential through constant currentsource Q₁₆₃, and resistors R₁₆₁ and R₁₆₂ form a voltage divider toprovide the necessary bias to the base of transistor Q₁₆₁. In order todetect the current I₂ that flows through diode circuit D₁, acurrent-mirror circuit is provided that is connected to the junctions ofthe three diodes making up the series-connected diode circuit D₁.Specifically, the current mirror is formed of transistors Q₁₇₁ and Q₁₇₂having their base leads connected across diode D_(1b) in diode circuitD₁. That is, the base of transistor Q₁₇₁ is connected to the junctionbetween diodes D_(1a) and D_(1b), and the base of transistor Q₁₇₂ isconnected to the junction between diodes D_(1b) and D_(1c). The currentmirror circuit 17 is connected to the input or base lead of differentialamplifier 16 by connecting the collector lead of transistor Q₁₇₂ to thebase lead of transistor Q₁₆₂ and the collector lead is also connected tothe bias supply voltage terminal T₁ through bias/load resistor R₁₇₁. Theoutput of transistor Q₁₆₂ is fedback to the input or base lead thereofby means of an emitter follower feedback circuit formed of transistorQ₁₈₁ and capacitor C₁₈₁. Specifically, transistor Q₁₈₁ has its emitterlead connected to a constant-current source 182, and the base leadconnected to the collector lead of transistor Q₁₆₂, through a capacitorC₁₈₁ in order to prevent oscillation.

Therefore, since the current-mirror circuit 17 operates as a currentmirror in conjunction with diode string D₁, then the equality thatcurrent I₁₇ =current I₂ can be made. Similarly, since the collectorcurrent I₁₆ of transistor Q₁₆₂ is also the collector current oftransistor Q₁₄₃, that is, the current-mirror circuit 14, then it can bestated that current I₁₄, which is the output of current mirror 14, isequal to the current I₁₆. If it is assumed that current I₂ increasesthen, based upon the above relation, current I₁₇ will similarly increaseand a increase in current I₁₇ will result in a decrease in current I₁₆,because the base voltage of transistor Q₁₆₂ must decrease when a currentI₁₇ increases. Accordingly, from the above relationship current I₁₄ willalso decrease and when current I₁₄ decreases, from equation (7)hereinabove, it is seen that current I₂ will also decrease and thusprovide negative feedback stabilization for the current I₂.

By choosing the values of resistors R₁₆₁ and R₁₇₁ to be equal, thereference current I_(r) can be made to equal the current I₂ and, since:

    I.sub.r =V.sub.cc /(R.sub.161 +R.sub.162)                  (14)

then

    I.sub.2 +V.sub.cc /(R.sub.161 +R.sub.162)                  (15)

Thus, equation (15) shows that the temperature characteristics of thecurrent I₂ will depend principally upon the temperature characteristicsof resistors R₁₆₁ and R₁₆₂.

On the other hand, the input signal current i₁, which is produced byapplying the input signal voltage to be compensated to the terminal T₁₁,can be written as:

    i.sub.1 =V.sub.1 /(R.sub.11 +R.sub.12)                     (16)

This equation (16) then also shows that the temperature characteristicsof the input signal current i₁ depend principally upon the temperaturecharacteristics of resistors R₁₁ and R₁₂. Accordingly, temperaturecharacteristics of currents I₂ and i₁ depend upon the temperaturecharacteristics of the respective resistor circuits. In modernintegrated circuit fabrication technology, the temperaturecharacteristics of resistors R₁₆₁, R₁₆₂, R₁₁, R₁₂ can be made equalquite easily during fabrication. Therefore, referring back to equation(13) it may be seen that "a" can be made equal to "b".

Therefore, equation (13) can be rewritten as follows: ##EQU2## equation(19) then represents that the temperature characteristics of thegamma-compensated current I₆ are not influenced by the number of diodesJ and K because the term (1+a) does not includes an exponentialcomponent of K or J. Equation (19) also shows that the temperaturecharacteristics of current I₆ are substantially equal to those ofcurrent I₁. Similarly, the above statements can also be made in regardto the output current.

Accordingly, the nonlinear circuit shown in FIG. 5 has agamma-compensation characteristic that is not influenced by the numberof diodes J and K that are essentially used to provide the desirednonlinear characteristics. Moreover, all of the output currents providedby the nonlinear circuit shown in FIG. 5 have the same temperaturecharacteristics so that the output currents can be added together toprovide any desired nonlinear output curve, because the sum of theseoutput signals will have the same temperature characteristics as each ofits components parts and is thus standardized.

Although illustrative embodiments of the present invention have beendescribed in detail hereinabove, it is to be understood that theinvention is not limited to such precise embodiments and that variouschanges and modifications can be effected therein by one skilled in theart without departing from the spirit or scope of the invention, asdefined in the appended claims.

What is claimed is:
 1. A circuit having a nonlinear responsecharacteristic for compensating an input signal, comprising:differentialamplifier means receiving said input signal at one input and producingan output signal therefrom at an output; a first diode circuit connectedbetween said differential amplifier means output and a referencepotential; first current-source means connected to supply a constantcurrent to said differential amplifier means and also to said firstdiode circuit; a second diode circuit connected to said differentialamplifier means output; current maintenance means connected to saidfirst diode circuit and to said second diode circuit for maintaining acurrent flow in said second diode circuit equal to a current flow insaid first diode circuit; a third diode circuit; and output circuitmeans having an input connected to said second diode circuit and beingconnected to said reference potential through said third diode circuitfor producing an output signal representing said input signal havingbeen nonlinearly compensated.
 2. A circuit having a nonlinear responsecharacteristic according to claim 1, in which said differentialamplifier means comprises a transistor pair and further comprises afourth diode circuit connected between respective collector circuitsthereof, said first diode circuit being connected to a collector circuitof one transistor of said pair and said output of said differentialamplifier means being taken at a collector circuit of the othertransistor of said pair, said input signal being fed to one base circuitand a feedback means being connected to the other base circuit of saidtransistor pair to maintain a balance between the collector circuits ofsaid transistors pair of said differential amplifier means.
 3. A circuithaving a nonlinear response characteristic according to claim 2, inwhich said feedback means comprises a difference amplifier having anadditive input connected to one of said collector circuits of saidtransistor pair and having a subtractive input connected to the other ofsaid collector circuits of said transistor pair for producing an outputsignal representative of a difference between said two inputs and beingfed to said other base circuit of said transistor pair forming saiddifferential amplifier means, and a capacitor connected to receive saidoutput signal of said difference amplifier and connected to saidreference potential.
 4. A circuit having a nonlinear responsecharacteristic according to claim 3, in which said input signal is avideo luminance signal and a clamping synchronization signal is fed toan additional input of said difference amplifier, whereby said outputsignal of said difference amplifier constitutes a clamping level outputin response thereto.
 5. A circuit having a nonlinear responsecharacteristic according to claim 4, in which said fourth diode circuitcomprises K series-connected diodes, where K is a positive integer.
 6. Acircuit having a nonlinear response characteristic according to claim 2,in which said fourth diode circuit comprises K series-connected diodesand in which said first diode circuit comprises (N-K) series-connecteddiodes, where N and K are positive integers.
 7. A circuit having anonlinear response characteristic according to claim 6, in which saidcurrent maintenance means is connected to a last one of said (N-K)series-connected diodes and is connected to said reference potential. 8.A circuit having a nonlinear response characteristic according to claim7, in which said current maintenance means comprises a current-mirrorcircuit including at least one current-mirror transistor having a baselead connected to said last diode of said first diode circuit and acollector lead connected to said second diode circuit and to the inputof said output circuit means.
 9. A circuit having a nonlinear responsecharacteristic according to claim 8, in which said output circuit meanscomprises a first output transistor having a base lead connected to thecollector lead of said current-mirror transistor and in which said thirddiode circuit comprises (J-1) series-connected diodes, where J is apositive integer.
 10. A circuit having a nonlinear responsecharacteristic according to claim 9, in which said current-mirrorcircuit of said current maintenance means includes a second currentmirror transistor having a base lead connected to the base lead of saidfirst current-mirror transistor and a collector lead connected to afifth diode circuit connected to said output of said differentialamplifier means, and said output circuit means further comprises asecond output transistor having a base lead connected to the collectorlead of said second current-mirror transistor and an emitter leadconnected to said reference potential through a sixth diode circuit. 11.A circuit having a nonlinear response characteristic according to claim10, in which said fifth diode circuit means includes (N-J-1)series-connected diodes, where J is a positive integer.
 12. A circuithaving a nonlinear response characteristic for providing gammacompensation to an input video luminance signal, comprising:first diodecircuit means; second diode circuit means; a differential transistoramplifier formed of a pair of transistors and having said input signalconnected to a base lead of a first transistor and having a collectorlead thereof connected to a reference potential through said first diodecircuit means, collector leads of the pair of transistors forming saiddifferential transistor amplifier being interconnected through saidsecond diode circuit means; a current source connected to supply aconstant current to said differential transistor amplifier and to saidfirst diode circuit means; a difference amplifier means having inputsconnected respectively to said collector leads of said pair oftransistors forming said differential transistor amplifier and providingan output difference signal fed to a base lead of a second transistor ofsaid pair of transistors forming said differential transistor amplifier;a capacitor connected to receive said output difference signal from saiddifference amplifier and being connected to said reference potential;current-mirror circuit means connected to said first diode circuitmeans; third diode circuit means connected to one of said collectorleads of said pair of transistors forming said differential transistoramplifier and to an output of said current-mirror circuit means, whereinsaid current-mirror circuit means operates to cause a current flowingthrough said third diode circuit means to equal a current flowingthrough said first diode circuit means; fourth diode circuit means; andoutput circuit means connected to said third diode circuit means andbeing connected to said reference potential through said fourth diodecircuit means for producing a circuit output signal that is anonlinearly compensated reproduction of said input video luminancesignal.
 13. A circuit having a nonlinear response characteristicaccording to claim 12, in which said difference amplifier means has anadditional input connected to receive a clamping synchronization signal,whereby said output difference signal constitutes a clamping leveloutput fed to said differential transistor amplifier.
 14. A circuithaving a nonlinear response characteristic according to claim 12, inwhich said first diode circuit means includes (N-K) series-connecteddiodes, said second diode circuit means includes (K) diodes, said thirddiode circuit means includes (N-J-1) series-connected diodes and saidfourth diode circuit means includes (J-1) series-connected diodes whereN, K, and J are positive integers.
 15. A circuit having a nonlinearresponse characteristic according to claim 14, in which saidcurrent-mirror circuit means is connected to a last one of said (N-K)series-connected diodes forming said first diode circuit means and isconnected to said reference potential.
 16. A circuit having a nonlinearresponse characteristic according to claim 15, in which saidcurrent-mirror circuit means comprises a first current-mirror transistorhaving a base lead connected to said last diode of said first diodecircuit means and a collector lead connected to said third diode circuitmeans and to the input of said output means.
 17. A circuit having anonlinear response characteristic according to claim 16, in which saidoutput circuit means comprises a first output transistor having a baselead connected to the collector lead of said first current mirrortransistor.
 18. A circuit having a nonlinear response characteristicaccording to claim 17, in which said current-mirror circuit meansincludes a second current-mirror transistor having a base lead connectedto the base lead of said first current-mirror transistor and a collectorlead connected to a fifth diode circuit means which is connected to saidcollector lead of said one of said pair of transistors forming saiddifferential transistor amplifier whereat said third diode circuit meansis connected and said output means comprises a second output transistorhaving a base lead connected to the collector lead of said secondcurrent-mirror transistor and an emitter lead connected to saidreference potential through a sixth diode circuit means.
 19. A circuithaving a nonlinear response characteristic according to claim 18, inwhich said sixth diode circuit means includes (J₁ -1) series-connecteddiodes, where J₁ is a positive integer.
 20. A gamma-compensation circuithaving a nonlinear response characteristic for providing gammacompensation to a video luminance signal, comprising:differentialamplifier means receiving said video luminance signal at one inputthereof for producing an output signal therefrom at an output; a firstseries-connected diode circuit connected between said differentialamplifier means output and a reference potential; current-source meansconnected to supply a constant current to said differential amplifiermeans and to said first series-connected diode circuit; a secondseries-connected diode circuit connected to said differential amplifiermeans output; current-mirror circuit means connected to an output ofsaid first series-connected diode circuit and to an output of saidsecond diode series-connected circuit for causing the current flowingthrough said second diode circuit to equal the current flowing in saidfirst series-connected diode circuit; and output means including a firstoutput transistor having a base lead connected to the junction of saidsecond series-connected diode circuit and said current-mirror circuitand having a third series-connected diode circuit connected between theemitter lead of said first output transistor and the referencepotential, whereby a gamma-compensated video luminance signal currentflows in the collector circuit of said first output transistor.